Organic synthesis Flashcards
Which reactions increase chain length?
- KCN with a halogenoalkane to produce a nitrile
- HCN with an aldehyde or ketone to produce a hydroxynitrile
- Friedel-Crafts alkylation to introduce a side chain to a benzene ring
What reactions decrease chain length?
- Soda lime with an acid or its salt to produce an alkane
- Iodoform with a methyl ketone to produce a carboxylic acid salt
What does miscible mean?
Liquids are completely soluble in each other at all concentrations.
How do you separate miscible liquids which do not decompose at or below their boiling temperature?
Distillation can be used to separate the product from the reaction mixture.
What is another use of distillation other than separating miscible liquids?
It is used to separate a volatile liquid from other substances in the mixture that are not volatile.
When is fractional distillation used?
When separating miscible liquids with boiling temperatures closer together.
How do you separate miscible liquids which decompose before or at their boiling temperature?
Distillation is carried out under reduced pressures or steam distillation is used, allowing the compounds to boil at lower temperatures.
How do you separate immiscible liquids?
Steam distillation is used, where steam is passed into the reaction mixture and the volatile compounds pass over with steam and condense in the receiving flask.
When is solvent extraction used?
When a compound has differing solubility in two immiscible solvents. Two different layers of a mixture can be separated using a separating funnel.
How do you separate insoluble solids?
Filtration is used to separate the solid from the liquid present. This can be carried out using a filter paper and a funnel. Once the solid is in the funnel it needs to be washed with an appropriate solvent and dried in the air or a drying oven.
Why is the use of fluted filter paper quicker than normal filter paper?
As the filtrate only needs to travel through one layer of filter paper and the paper only touches the funnel at the folds.
How do you separate soluble solids from solution?
The products are obtained by crystallisation.
What do you need to do if the solution obtained from crystallisation is coloured?
The solution is boiled with decolourising charcoal, and filtered hot to remove the charcoal that contains the absorbed colour.
What are the steps for recrystallisation when the solute is impure?
- Dissolve the solute in a minimum volume of hot solvent
- Filter hot, if necessary, to remove insoluble impurities
- Allow to cool
- Filter
- Wash the solid with a small amount of ice cold solvent
- Dry at a temperature below its melting temperature
What is polymerisation?
The joining together of a large number of monomer molecules.
What types of molecules undergo addition polymerisation?
Alkenes, where the C=C bond is lost.
What is condensation polymerisation?
A large number of monomer molecules joining together with the loss of small molecules.
How can you recognise addition polymerisation?
If the monomer is an alkene
If no small molecules is lost and the polymer is the only product
If the chain only consists of carbon atoms
How can you recognise condensation polymerisation?
If the monomers contain -NH2, -COOH or -OH
If the chain contains an amide or ester linkage
What is PET?
A polyester made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid.
What are the properties of PET?
It is a good insulator so is made into synthetic fibres, but does not easily biodegrade.
What is Nylon 6,6?
A polyamide made from two monomers with six carbon atoms.
What is Kevlar?
A polyamide made from benzene-1,4-dioic acid and benzene-1,4-diamine.
What are the properties of Kevlar?
It is fire retardant and is five times stronger than steel, so it is used in bulletproof vests.
What are indications that a compound is impure?
If it has a lower melting temperature than expected or melts over a range of temperatures.
How can you obtain a melting temperature for aldehydes and ketones?
React with 2-4-DNPH to give a derivative, then the melting temperature can be compared with textbook values to identify the starting aldehyde or ketone.
What are two methods to obtain melting temperatures of compounds?
Electrical heating
Heating bath
Describe how to find melting temperature of a compound using a heating bath
A sample is placed in a capillary tube that is attached to a thermometer by a rubber band which are both placed into the heating bath. The mixture is gently heated and stirred.
What does the number of peaks tell us in high resolution 1H NMR?
The number of different proton environments.
What do the peak areas tell us in high resolution 1H NMR?
The number of hydrogens in each environment.
Why does splitting of peaks occur in high resolution 1H NMR?
As the magnetic environment of a proton or protons in one group is affected by the magnetic environment of neighbouring groups.
What is the process of signal splitting called?
Spin-spin coupling.
What does the peak splitting tell us in high resolution 1H NMR?
The number of protons adjacent to the environment measured PLUS ONE.
What is chromaotgraphy?
A technique that is used to separate substances from a mixture by their slow movement, at different rates, through or over a stationary phase.
Describe paper chromatography
The stationary phase is water trapped in the cellulose fibres of the paper. Spots of the starting materials in solution are placed at the bottom of the paper. The solvent front then rises up the paper, separating the mixture. The paper is then removed and dried.
Describe TLC
The stationary phase is a layer of SiO2 or Al2O3 coated onto the plate. Spots of the starting materials in solution are placed at the bottom of the plate. The solvent front then rises up the plate, separating the mixture. The plate is then removed and dried.
How do you calculate the Rf value?
Distance moved by spot / Distance moved by solvent front
What happens if the use of a particular solvent does not completely separate the spots?
The dried chromatogram is rotated through 90° and a different solvent is used, called two-way separation.
What techniques can be used if chromatogram spots are colourless?
- If amino acids are used, spray with a solution of ninhydrin and warm to produce blue-purple spots
- Shine UV light onto plate if compounds are fluorescent
- Coat plate with fluorescence where spots are dark when UV is shone on them
What are the advantages of TLC over paper chromatography?
TLC is faster and the thin layer on the plates can be made from a variety of materials.
Describe gas-liquid chromatography
A gaseous mixture is passed ‘through’ liquid particles supported on an inert solid. The gaseous mixture is swept into the column by a carrier gas where there is a liquid stationary phase.
What is the retention time in GLC?
The time taken from the sample entering the injection part until it reaches the detector.
What does separation of the mixture depend on in GLC?
The volatility of the compound
The column length
The column temperature
The flow rate of the carrier gas
Describe high performance liquid chromatography
The column is packed with uniform size solid particles and the mixture sample is dissolved in a suitable solvent. The solution is then forced through the column at high pressure.
What is high performance liquid chromatography used for?
Compounds that vaporise at high temperatures where they may start to decompose.
